The plot could start with Shrek going into a haunted forest on Halloween. There, he meets a witch who has been cursing the forest animals. Shrek decides to stop her. He tries to reason with her at first, but she doesn't listen. So, Shrek has to use his strength and wits to outsmart the witch. Maybe he could find a magical object in the forest that can reverse the witch's spells. In the end, he manages to free the animals and send the witch packing.
A possible plot is that Shrek hears rumors of a witch on Halloween who is stealing the dreams of the villagers. Shrek goes to investigate. When he finds the witch's lair, she captures him. The witch plans to use Shrek's big, strong body to carry more of her dream - stealing devices. Shrek manages to escape and finds a way to destroy the devices. Then he confronts the witch. The witch tries to use her magic on Shrek, but he uses his own natural ogre defenses. In the end, Shrek scares the witch so much that she leaves the village and never returns.